Dust particles generated during the process of producing semiconductors degrade the products. Therefore, semiconductor manufacturing equipments are normally equipped with a particle counter for counting in real time the number of particles generated in the process chamber.
The particle counter is generally provided in an exhaust duct from the process chamber, and includes: a light source for emitting a laser light to the measurement area in the exhaust duct; a detector for detecting the light scattered by a particle on which the laser light is irradiated; a discriminator for determining whether or not a particle exists by comparing the detection signal of the detector with a predetermined specific discrimination threshold; and other units.
The detector detects not only the light scattered by particles that pass the measurement area but also the light scattered by molecules of gas such as oxygen (or air), nitrogen, and other elements existing as a background gas in the measurement area. This is because molecules of gas such as oxygen, nitrogen, and other elements are very small particles and scatter light when irradiated.
When the background noise of the scattered light due to the gas molecules as just described is large, an accurate discrimination is difficult between the signal of the scattered light by real particles and that of the background light noise. If the gas molecules existing in a measurement area are further abundant, the intensity of the background light may be stronger than that of the light scattered by the smallest particles to be measured, which prevents the detection of small-size particles.
Given such factors, a method to decrease the background light noise has been proposed in which scattered lights are detected by a detector composed of a large number of sensing elements, i.e. having a large number of pixels (refer to Patent Document 1). This is based on the principle that narrowing the detection area of one sensing element reduces the background light noise, so that small-size particles can be detected.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H06-26823